The present invention relates to a radio communication system and further relates to primary and secondary stations for use in such a system and to a method of operating such a system. While the present specification describes a system with particular reference to the emerging Universal Mobile Telecommunication System (UMTS), it is to be understood that such techniques are equally applicable to use in other mobile radio systems.
There are two basic types of communication required between a Base Station (BS) and a Mobile Station (MS) in a radio communication system. The first is user traffic, for example speech or packet data. The second is control information, required to set and monitor various parameters of the transmission channel to enable the BS and the MS to exchange the required user traffic.
In many communication systems one of the functions of the control information is to enable power control. Power control of signals transmitted to the BS from a MS is required so that the BS receives signals from different MS at approximately the same power level, while minimising the transmission power required by each MS. Power control of signals transmitted by the BS to a MS is required so that the MS receives signals from the BS with a low error rate while minimising transmission power, to reduce interference with other cells and radio systems. In a two-way radio communication system power control may be operated in a closed or open loop manner. In a closed loop system the MS determines the required changes in the power of transmissions from the BS and signals these changes to the BS, and vice versa. In an open loop system, which may be used in a TDD system, the MS measures the received signal from the BS and uses this measurement to determine the required changes in the MS transmission power.
An example of a combined time and frequency division multiple access system employing power control is the Global System for Mobile communication (GSM), where the transmission power of both BS and MS transmitters is controlled in steps of 2 dB. Similarly, implementation of power control in a system employing spread spectrum Code Division Multiple Access (CDMA) techniques is disclosed in U.S. Pat. No. 5,056,109.
In considering closed loop power control it can be shown that for any given channel conditions there is an optimum power control step size which minimises the required Eb/N0 (energy per bit/noise density). When the channel changes very slowly the optimum step size can be less than 1 dB, since such values are sufficient to track changes in the channel while giving minimal tracking error. As the Doppler frequency increases, larger step sizes give better performance, with optimum values reaching more than 2 dB. However, as the Doppler frequency is further increased there comes a point where the latency (or update rate) of the power control loop becomes too great to track the channel properly and the optimum step size reduces again, perhaps to less than 0.5 dB. This is because the fast channel changes cannot be tracked so all that is needed is the ability to follow shadowing, which is typically a slow process.
Because the optimum power control step size can change dynamically it may improve performance if the BS determines an appropriate power control step size for use in uplink transmissions from MS to BS and downlink transmissions from BS to MS, and informs the MS accordingly. An example of a system which may use such a method is the UMTS Frequency Division Duplex (FDD) standard, where power control is important because of the use of CDMA techniques. Although improved performance can be obtained by having a small minimum step size, for example 0.25 dB, this will significantly increase the cost of a station. However, if a station does not have to implement the minimum step size then it may not be able to implement the requested step size.
A further problem may occur in a system in which implementation of some power control step sizes by a station is optional. For example, in a system operating according to the UMTS specification a BS may use a plurality of different power control step sizes when changing the downlink transmission power, for example the four step sizes 0.5 dB, 1 dB, 1.5 dB and 2 dB. However, it may be the case that only implementation of a 1 dB step size is mandatory. In some circumstances it may be desirable to ensure that different BSs behave in a similar way. For example, during soft handover, a MS engages in communication with a plurality of BSs (known as the xe2x80x9cactive setxe2x80x9d of BSs) to determine to which BS, if any, it should transfer. It is therefore necessary to avoid the transmission power of the BSs in the active set from diverging significantly. This is best achieved if the BSs in the active set change their transmission power in similar ways, for example by using similar power step sizes in response to received power control commands.
If two BSs are in soft handover with a MS which is moving at a speed such that 1.5 dB step sizes are optimal, but only one of the BSs supports 1.5 dB steps, optimum power control of both BSs is not possible. The network has to choose between instructing the BSs to use different step sizes, so that the optimum step size can be used by the BS supporting it (with the risk that the transmit powers of the two BSs will diverge significantly), or instructing both BSs to use the same non-optimal step size (e.g. 1 dB or 2 dB) in order to avoid undue divergence of transmit power. Clearly, neither choice is optimal.
An object of the present invention is to enable selection of optimum power control step sizes without requiring all stations to implement the same set of step sizes.
According to a first aspect of the present invention there is provided a radio communication system having a communication channel between a primary station and a secondary station, one of the primary and secondary stations (the transmitting station) having means for transmitting power control commands to the other station (the receiving station) to instruct it to adjust its output transmission power, wherein the receiving station has emulation means for emulating an unsupported power control step size by a combination of power control steps of at least one supported size.
According to a second aspect of the present invention there is provided a primary station for use in a radio communication system having a communication channel between the primary station and a secondary station, the primary station having means for adjusting its output transmission power in steps in response to power control commands transmitted by the secondary station, wherein emulation means are provided for emulating an unsupported power control step size by a combination of power control steps of at least one supported size.
According to a third aspect of the present invention there is provided a secondary station for use in a radio communication system having a communication channel between the secondary station and a primary station, the secondary station having means for adjusting its output transmission power in steps in response to power control commands transmitted by the primary station, wherein emulation means are provided for emulating an unsupported power control step size by a combination of power control steps of at least one supported size.
According to a fourth aspect of the present invention there is provided a method of operating a radio communication system having a communication channel between the primary station and a secondary station, the method comprising one of the primary and secondary stations (the transmitting station) transmitting power control commands to the other station (the receiving station) to instruct it to adjust its power in steps, wherein the receiving station emulates an unsupported power control step size by a combination of power control steps of at least one supported size.
The present invention is based upon the recognition, not present in the prior art, that emulation of small power control step sizes by a station can provide good performance.